energize vs let up

energize

verb
  • To invigorate; to make energetic. 

  • To supply with energy, especially electricity; to turn on power to (something). 

let up

verb
  • To cease or stop. 

  • To cease or stop (doing something intense). 

How often have the words energize and let up occurred in a corpus of books? (source: Google Ngram Viewer )